First-class Temporal integration for Google ADK

**Is your feature request related to a problem? Please describe.**
ADK apps can persist state (e.g., via `SessionService`) but not the execution itself. If the host dies mid-turn or a step fails transiently (rate limits, network outage, restarts), you lose in-flight progress and must re-run expensive steps. Teams are hand-wiring [Temporal](https://temporal.io/) around ADK to get crash-safe, resumable execution, but each team rebuilds the same plumbing (retries, backoff, state handoff, observability), leading to inconsistent patterns and higher ops burden.

**Describe the solution you'd like**
A `TemporalRootAgent` (or equivalent) that starts a Temporal Workflow per ADK session/run and invokes ADK steps as Activities.

**Describe alternatives you've considered**
- Replace ADK’s Runner: Brittle; Runner is the core event engine and not intended to be swapped.
- Single coarse “run everything” Activity: Loses mid-flow retries and HITL gates; failures force full reruns.
- [Orchestrating A2A-exposed agents from a Temporal workflow](https://medium.com/google-cloud/orchestrating-multi-agent-systems-a-deep-dive-into-google-adk-a2a-protocol-and-temporal-b13a18638890): Require to duplicate orchestration logic inside the agent and in the workflow, fragments observability.

**Additional context**
Temporal + OpenAI Agents integration announcement: [https://temporal.io/blog/announcing-openai-agents-sdk-integration](https://temporal.io/blog/announcing-openai-agents-sdk-integration)